The present invention relates to a photo detecting element and more particularly to a super high speed and high sensitive photo detecting element fabricated by multilayer of thin film high temperature superconducting material and thin film ferromagnetic material.
Recently, there have been actively conducted many research and development works for creating a network system that can effectively process a huge amount information at high speed by combining the fiber optics communications with ultra high speed switches. A superconducting switching element using single flux quantum (SFQ) has been developed as an ultra high speed switching element. This type switching element has a processing speed of about 100 Gb/s. Many attempts of coupling fiber optics communications with SFQ type ultra high speed switching elements usually have been made by using optical fibers and semiconductor photo detecting elements (photo diodes).
However, the semiconductor type photo detecting elements have not sufficient speed in response to radiation and further involves a problem that they can not be formed on the same substrates on which the SFQ switching elements made of superconducting material are formed. In other words, the separate arrangement of the photo detecting element from the SFQ switching element elongates the transmission path of data between them, which is associated with decrease of the SN ratio and the operation speed of the system. On the other hand, the high temperature superconductor has received a great deal of attention since it has a very excellent response to radiation and very suited to application as a superconducting photo detecting element (as described by Hegmann et. al., Appl. Phys. Lett., Vol. 67, 1995, pp. 285).
The examination has been made to develop new material by multilayer of high temperature superconducting material and perovskite type manganese oxides since both materials are similar to each other in crystal structure and have close lattice constants (Chahara et al., Appl. Phys. Lett. Vol. 62, 1993, pp. 780).
Recently, it has been reported that a three terminal device having a large amplification can be created by sandwiching a film of perovskite type insulating material between a film of superconducting material and a layer of perovskite type manganese oxide being ferromagnetic material having a large spin polarization f actor and by forcing spin polarized guasiparticles to inject from the ferromagnetic material into the superconducting material (Vas'koetal., Phys. Rev. Lett., Vol. 78, 1997, pp. 1134).
In view of the above, the present invention was directed to development of a photo detecting element made of superconducting material. Many attempts have also been made to develop, in particular, oxide high temperature superconducting material which was expected to realize a very short response time to radiation, which was estimated at 100 Gb/s or higher. However, the high temperature superconductor had a problem that it has small voltage sensitivity to radiation.